Multimedia-enabled cellular phone systems enable users of phones and other terminals to receive multimedia content, including objects such as pictures, music, video, and executable programs. Multimedia Messaging Service (MMS) enables phone users to send and to receive multimedia messages that encapsulate multimedia content. Users may also download multimedia content to their phones from a content download platform (CDP). Operators of cellular phone systems may generate revenue by charging users for the receipt and/or transmission of multimedia content. Operators may also compensate or share revenue with content providers or users who contribute content. When content is forwarded many times between users, the propagation may be referred to as “viral distribution.”
For example, U.S. Patent Application Publication 2005/0021394, whose disclosure is incorporated herein by reference, describes a method and system for distributing a multimedia object. A server receives a request to buy the multimedia object from a client device. After authorizing the request, an identifier for the object is forwarded to a file sharing network. A node that is able to distribute the object to the client signals a positive response to the server and subsequently transmits the object to the client. The operator of the node is then rewarded for distributing the object in this fashion.
In one embodiment, the request is forwarded by obtaining a “fingerprint” for the multimedia object and submitting a query comprising the fingerprint to a node in the file sharing network. A fingerprint of this sort is a representation of the most relevant perceptual features of the object in question. The fingerprints of a large number of multimedia objects along with their associated respective metadata, such as the title, artist, genre and so on, are stored in a database. The metadata of a multimedia object are retrieved by computing its fingerprint and performing a lookup or query in the database using the computed fingerprint as a lookup key or query parameter. The lookup then returns the metadata associated with the fingerprint.
U.S. Patent Application Publication 2006/0075237, whose disclosure is incorporated herein by reference, describes a method and arrangement for extracting a fingerprint from a multimedia signal, particularly an audio signal, which is invariant to speed changes of the audio signal. The method comprises extracting a set of robust perceptual features from the multimedia signal, for example, the power spectrum of the audio signal. A Fourier-Mellin transform converts the power spectrum into Fourier coefficients that undergo a phase change if the audio playback speed changes. Their magnitudes or phase differences constitute a speed change-invariant fingerprint. By a thresholding operation, the fingerprint can be represented by a compact number of bits.
Various methods have also been proposed for detecting replicated images notwithstanding changes such as image format conversion, resampling, and transformations such as translation, scaling and rotation. Some approaches use feature vectors, as described, for example, by Chang et al., in “RIME: A Replicated Image Detector for the World-Wide Web,” Proceedings of SPIE Symposium on Voice, Video and Data Communications (Boston, Mass., November, 1998), pages 58-67, which is incorporated herein by reference. RIME characterizes each image using a Discrete Wavelet Transform (DWT) for each of the three color components. Since each image is thus represented by a multi-dimensional feature vector, searching for replicas of an image is performed by searching for vectors in the neighborhood of the given vector.